Engines may operate with a variable number of strokes in a combustion cycle. For example, an engine may be configured to operate in a first mode with cylinders carrying out combustion in a two-stroke combustion cycle, and further to operate in a second mode with cylinders carrying out combustion in a two-stroke combustion cycle. The engine may transition, during engine operation, between these modes with various valve systems, such as cam switching actuators, electric cylinder valve actuators, etc.
One such example is provided by Kamamura in U.S. Pat. No. 5,022,353. Herein, the variable-cycle engine is configured to operate in a two-stroke mode when the engine speed is lower than a predetermined threshold and operate in a four-stroke mode when the engine speed is above the threshold. In this approach, at lower engine speeds and in the two-stroke mode, a smooth engine rotation and high torque output may be attained, while at higher engine speeds and in the four-stroke mode, higher engine efficiency and lower fuel consumption may be achieved.
The inventors herein have recognized some issues with the above approaches and approaches of that kind. While under some conditions, and for some transitions, the above approach may be used to advantage, there may be specific instances in which degraded operation may occur. For example, the inventors herein have recognized that due to transient turbocharger effects, such as surge, etc., the above approach may schedule a transition to two-stroke operation when insufficient boost is currently available. For example, boost may be temporarily dropped and in the process of recovering, when a transition is scheduled. In that case, the initial operation with two-stroke combustion may be degraded due to excess residuals left in the combustion chamber, thus degrading combustion and potentially generating an engine misfire, for example. Likewise, other conditions can occur where, at least transiently, insufficient boost is available to effectively support two-stroke combustion operation. However, the inventors have also recognized that such conditions may be particular to certain combustion modes.
The above issues may be at least partially addressed by a method of operating an engine having at least a cylinder, the method comprising: transitioning from operating the cylinder with a first number of strokes per combustion cycle to a second, lesser, number of strokes per combustion cycle in response to boost pressure rising above a threshold boost value. For example, the cylinder may transition from four-stroke combustion cycles to two-stroke combustion cycles, and the method may further include generating the increased boost via operation of an electric machine coupled to a turbocharger of the engine. In this way, transitions to the second, lesser, number of strokes may be enhanced as sufficient boost can already be present.
In another example, the method comprises: boosting intake air delivered to the cylinder; operating the cylinder with four strokes per combustion cycle; during at least the operation with four strokes per combustion cycle, adjusting boost of the intake air responsive to operating conditions; in response to a selected condition, transitioning from the operation with four strokes per combustion cycle to two strokes per combustion cycle only when a boost is greater than a threshold boost amount and when cylinder peak combustion pressure is greater than a threshold peak cylinder pressure; adjusting throttling, spark timing, and the boost during the transition from the operation with four strokes per combustion cycle to two strokes per combustion cycle; and transitioning from the operation with two strokes per combustion cycle to four strokes per combustion cycle based on engine speed and torque requested.
In this way, it is possible to take into account different constraints in entering two-stroke combustion cycles as compared to four-stroke combustion cycles.
It should be understood that the background and summary above is provided to introduce in simplified form a selection of concepts that are further described in the detailed description. It is not meant to identify key or essential features of the claimed subject matter, the scope of which is defined uniquely by the claims that follow the detailed description. Furthermore, the claimed subject matter is not limited to implementations that solve any disadvantages noted above or in any part of this disclosure.